The invention relates to an element, in particular a photopolymer unit, with a rear layer and a top layer which are joined to one another to form a closed bag in which a precursor of a plastic which can be polymerised by electromagnetic radiation is contained, in particular a photopolymer, a method of producing a printing plate for a stamp with a surface provided with a relief pattern using an element with a top layer and a rear layer which are joined to one another to form a closed bag in which a precursor of a plastic which can be polymerised by electromagnetic radiation is contained, and a device for producing a printing plate for a stamp with a relief pattern which comprises a production apparatus with two plates which are transparent to light in particular, and at least one radiation source for emitting electromagnetic radiation, in particular at least one UV light source.
The production of printing plates or text plates of stamps involving the selective curing of light-cured liquid or pasty plastics under the effect of radiation is already known from the prior art. To this end, the liquid photopolymer (prepolymer) is enclosed in a bag made from a thin transparent plastic film. Two sheets are usually welded to one another at their edges to obtain a polymer unit in the form of a flat bag. To enable a printing plate for a stamp to be produced from such a polymer unit or polymer pack, the polymer unit together with a mask—usually in the form of a photographic negative—are placed between two parallel glass plates of an appropriate production or lighting apparatus, and the mask lies between the bottom transparent plate and the polymer unit. A continuous base layer is formed due to the effect of radiation from above and curing of the polymer is induced as a result, and a relief-type structure corresponding to the shape of the mask and joined to the base layer is formed due to the effect of radiation from underneath. Once the photopolymer unit subjected to radiation treatment and cured in this manner is removed from the lighting apparatus, the cured plate is released from the bag and the non-cured prepolymer is washed off the non-illuminated areas. Such methods of producing these photopolymer units for producing printing plates for stamps are already known from JP 60141148 A (Maeda et al.) and EP 0 607 106 A1 (De Caria).
US 2003/0039915 A1 (Holt) discloses photopolymer packs for producing printing plates for stamps and the films used to make the bag of the photopolymer bags is made from a material which can be detached from the cured polymer. This being the case, the material of the films specifically contains polyolefins. Also disclosed by this document is the fact that laminates of polyethylene and polyethylene terephthalate are used for the films.
Another aspect which is known about the prepolymer packs known from the prior art is that when producing printing plates with the described method, the quality is very often detrimentally affected by irregularities or deformations in the contours of the relief pattern of the printing plates. A major reason for this appears to be the fact that the films used have a tendency to form creases as they are placed in the lighting apparatus and creases tend to form when the bag is being produced from the two films, e.g. by welding, especially if the two films are made from different polymers with different heat expansion properties.